The information storage industry is driven by market demands to increase continually the capacity and performance of devices for storing information. One of the needs is distribution of information (spatial communication) to various locations and retention of information (temporal communication) to be accessed at a later time. One popular application for information storage is storage of video information, such as movies, TV shows, and home videos. Yet another popular application is storage of music information. Another application is the storage and distribution of software to end-users. Driven by and reflecting this market demand, a variety of storage formats have been introduced into the market to fill various needs.
There are numerous methods of storing information, such as through printed matter (e.g., books and magazines), semiconductor-based RAM and FLASH memories, magnetic-based MRAM or bubble memories, magnetic-based Winchester-type disc drives, optical storage using phase-change or prefabricated or “burned” media, and holographic storage, among others. There are certain advantages and disadvantages of each type and, over time, certain types of storage tend to dominate certain applications.
The compact disc (“CD”), a type of OSM, was introduced in the 1970s and soon became a popular method for storage and distribution of music information due to certain advantages it held over the then state-of-the-art (cassette tapes and LP records). The CD medium was also adopted for storage and distribution of computer software due to certain advantages it held over the then state-of-the-art (floppy discs). Further advances in media and CD recorder/player technology, types of readback devices, allowed companies and consumers to record their own CDs, using several different formats available, to store information of many types from music and video, to pictures and images, to software and data. The capacity of CDs varied but was on average around 600 MB per disc. This was sufficient for many applications, but was not adequate to store a motion picture without considerable compression.
The DVD (sometimes styled as Digital Video Disc or Digital Versatile Disc although the exact expansion (if any at all) of the acronym is not generally agreed on), another type of OSM, was introduced in the 1990s and quickly became popular for distribution of pre-recorded video information, such as movies and extra features. The DVD format also allows for storage and distribution of software and other forms of data. Further advances in DVD media and DVD recorder/player technology, another type of OSD, allowed companies and consumers to record their own DVDs to store information of many types from music and video, to pictures and images, to software and data. The capacity of DVDs vary, some DVDs have a capacity of about 4.7 Gbytes per DVD. This is sufficient to store a full length motion picture feature plus other information that is of interest and benefit to consumers. The adoption of DVD technology into the market was one of the most rapid market penetration stories of our time.
New technologies are now emerging to store even more data on an optical storage medium. For example, two new competing formats (others may emerge) are popularly referred to as Blu-Ray and HD-DVD. These formats can each store over 15 Gbytes per disc. This enables the storage and distribution of an HDTV-format movie on a single disc. Information storage using the principles of holography is also under development. Other improvements and formats will doubtless be introduced from time to time in this competitive market.
In general conceptual terms, the physical structure and operational principles of most optical storage methods is similar. FIG. 1 is a partial side section view of an OSM 10. For the purposes of illumination but not by way of limitation, binary data is encoded and recorded onto the disc by differences in the height of the recording layer (generally called “lands” 12 and “pits” 14). Data may also be recorded using changes in the phase of the material, or other methods. The OSM includes a substrate 16 of acrylic or other material. In a phase-change based OSM, a layer of the appropriate material is included in the substrate. In an OSM that uses difference in height, the pits and lands are encoded in the substrate. A protective coating 18, such as a polycarbonate, is applied over the substrate. The coating is optically transparent, at least for the wavelength of the laser used to read the data encoded from the pits and lands. An aluminized layer 20 may also be applied to the substrate over the pits 14 and lands 12. The aluminized layer improves reflection of the laser from the pits and lands. A label 22 may be on the substrate 18.
The laser is used to scan the disc and read back the data by detecting the variation in the reflected light. The data is stored in microscopic grooves or “tracks” running in a spiral around the disc. The OSD uses laser beams to scan these grooves, where minuscule reflective bumps (the lands) and non-reflective holes (the pits) aligned along the grooves modulate the laser signal which, when properly decoded, represents the zeros and ones of digital information.
DVD technology writes in smaller “pits” to the recordable media than CD technology. Smaller pits mean that the drive's laser must produce a smaller spot. DVD technology achieves this by reducing the laser's wavelength from the 780 nanometer (“nm”) infrared light used in standard CD drives to about 625 nm-650 nm red light.
Smaller data pits allow more pits per data track. The minimum pit length of a single layer DVD-RAM is 0.4 micron as compared to 0.834 micron for a CD. Additionally, DVD tracks are closer together, allowing more tracks per disc. Hence, track pitch—the distance from the center of one part of the spiral information or “track” to the adjacent part of the track—is smaller. On a 3.95 GB DVD-R, track pitch is 0.8 microns; CD track pitch is 1.6 microns. On 4.7 GB DVD-R media, an even smaller track pitch of 0.74 microns helps boost storage capacity.
These narrow tracks require special lasers for reading and writing—which can't read CD-ROMs, CD-Rs, CD-RWs, or audio CDs. DVD-ROM drive makers solved the problem by putting two lasers in their drives: one for DVDs, the other for CDs. To facilitate the focusing of the laser on smaller pits, DVD media uses a thinner plastic substrate than do CDs. Further, DVD media has a thinner protective coating that the laser must pass through to reach the pits to record or read data than does CD media. This reduction originally resulted in discs that were 0.6 mm thick—half the thickness of a CD. Even single-sided DVDs have two substrates, even though one isn't capable of holding data. Double-sided discs with two data surfaces must be turned over to read data on each side. In other OSMs, the information can be stored as phase changes in the media, dye changes, or in the direction of magnetization in a magneto-optical storage medium, among others.
In various OSM, then, the data layer is protected by a protective surface 18 that is substantially optically transparent. In CDs and DVDs, it is typically a polycarbonate material. One of the significant problems that current users of optical storage media face is damage to the OSM protective surface. This can scatter or change the behavior of the reflected or transmitted light to the point that the data can no longer be read or written or both. The OSM error correction coding (“ECC”) can handle errors of a certain size, depending on the OSD, but errors larger than that threshold cause the OSD to be unable to read or write through the damage. In DVD players, this can be manifested as skipping, freezing, or an inability to even recognize the DVD's presence. In CD players, it can manifest itself as a high-pitched and annoying click, skipping, freezing, or an inability to even recognize the CD's presence. The frequency of this damage has been growing year over year as the rapid market penetration of OSM has reached relatively unsophisticated consumers (e.g. children) who do not treat the fragile protective surface with proper care.
Conventional methods exist to identify and alleviate problems reading or writing data to an OSM. Such conventional materials typically involve checking a data stream read from the OSM for errors, error correction code circuitry, and monitoring read retry requests. What is needed is a way to analyze the protective surface of an OSM. What is also needed is a way to determine whether data can be successfully read as well as written to an OSM. These and other needs are addressed by implementations and aspects of the present invention, as set forth in further detail below.